1. Field of the Invention
The present invention relates to turbine rotors and in particular it relates to high pressure and low pressure integrated type turbine rotors used in steam turbines employed in thermal electric power generation.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Conventionally, as one type of turbine rotor for steam turbines for thermal electric power generation, high pressure and low pressure integrated turbine rotors utilizing integrated materials from the high pressure part to the low-pressure part have been known. The steam turbine is exposed to high-temperature and high-pressure steam on the side of its steam inlet. As the end portion is being approached, the temperature and pressure of steam decrease, so that the steam turbine is exposed to steam that has a highly expanded volume. Therefore, in the high-pressure part, the turbine blades are short in length and the stress applied to the turbine rotor is relatively small, and thus the diameter of the turbine rotor may be small. On the other hand, in the low pressure part, to receive the force exerted by a larger amount of steam, the length of the turbine blades must be large and the diameter of the turbine rotor must be large, resulting in a large stress being applied to the turbine rotor. Therefore, the characteristics required for the high pressure and low pressure integrated type rotors are high temperature strength, in particular excellent creep strength at the high-pressure part, and on the other hand, at the low pressure part, mechanical strength and excellent toughness at ordinary temperature.
Conventionally, as examples of heat-resistant steels for use in high pressure and low pressure integrated type turbine rotors, CrMoV steels, which belong to low-alloys, and 12Cr steels, which belong to high-Cr steels, have been exclusively used (see Japanese Patent Applications, First Publications (Kokai), Nos. Sho 60-165359 and Sho 62-103345). A process for obtaining a turbine rotor having creep properties and toughness simultaneously has been proposed, in which a CrMoV based steel species is processed into a turbine rotor member and the high-pressure and low-pressure parts of a single turbine rotor are separately heat treated under different conditions. For example, Japanese Patent Application, First Publication (Kokai), No. Hei 5-195068 discloses a process for obtaining a high pressure and low pressure integrated type turbine rotor having creep strength at high temperatures and toughness simultaneously, in which the high pressure part of a rotor member is quenched after heating at a temperature higher than the low pressure part and then the whole rotor member is tempered at a predetermined temperature. Japanese Patent Application, First Publication (Kokai) No. Hei 8-176671 discloses a process for obtaining a high pressure and low pressure integrated turbine rotor having excellent creep properties at high temperatures and toughness simultaneously, in which a rotor member is normalizing-treated at 1100 to 1150xc2x0 C. and pearlite-transformed, further normalizing-treated at 920 to 950xc2x0 C., the high pressure part and low pressure part are quenched at different temperatures, and then the whole rotor member is tempered.
However, in recent years, further improvement in the energy efficiency has been desired, and there has been a trend that the temperature and the amount of steam introduced into turbines is increased, resulting in much stricter characteristics being required for turbine rotors. Therefore, rotors of a conventional type are insufficient in mechanical properties at high temperatures, particularly in terms of creep strength, at their high-pressure parts. Accordingly, the need for developing a material that is durable in use at higher steam temperatures has been growing. On the other hand, for low-pressure parts, developing a material that is durable to stronger stresses and has increased toughness has become necessary.
Conventionally, a CrMoV steel is used after quenching the CrMoV steel heated to a temperature of about 950xc2x0 C. A higher heating temperature before quenching results in a higher strength of the material because precipitation of a pro-eutectoid ferrite phase, which is soft, is inhibited, and dissolution of the strengthening elements in a solid solution is promoted. However, another problem arises in that a higher heating temperature before quenching causes creep embrittlement of the material. Therefore, the heating temperature before quenching cannot be raised. Although attempts have been made in which various alloy elements were additionally used and heat treatments have been devised in order to inhibit the creep embrittlement, a satisfactory material has not yet been obtained.
A higher temperature before quenching causes a problem that coarsening of crystal grains is promoted and thus the toughness of the material deteriorates. In view of this, the temperature before quenching could not be elevated to 1000xc2x0 C. or more. Thus, to satisfy the high temperature strength and brittleness of a CrMoV steel simultaneously involves the difficulty that inconsistent heat treatment conditions are used in the production of the steel. As a result, no satisfactory turbine rotor suitable for large volume steam turbines for use at high temperatures has been obtained.
Accordingly, an object of the present invention is to provide a heat-resistant steel which can be quenched after heating to a higher temperature, has a toughness equivalent to or higher than that of a conventional CrMoV steel, and has excellent creep properties at high temperature such as a high creep rupture property, according to a creep test on an unnotched test piece, and inhibition of creep embrittlement. Another object of the present invention is to provide a turbine rotor comprising this novel heat-resistant steel.
In order to achieve the above objects, the present inventors have diligently carried out research, and found that impurities greatly affect the properties of a steel at high temperatures, particularly the creep embrittlement resistance. As a result, the present inventors found that a high pressure and low pressure integrated type turbine rotor which can be quenched after heating to a high temperature between 980xc2x0 C. and 1100xc2x0 C., and having excellent creep strength at its high pressure part, such as not being subject to creep embrittlement, and a high toughness at its low pressure part can be obtained not only by mixing alloy components with predetermined proportions, but also by minimizing the amount of trace impurity elements which are harmful, such as phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony. The present inventors have thus achieved the present invention.
The high-pressure part of the high pressure and low pressure integrated type turbine rotor has excellent high temperature properties with a creep rupture time of 3000 hours or longer, according to a creep test on an unnotched test piece, under specific conditions of a temperature of 600xc2x0 C. and a stress of 147 MPa, and a creep rupture time of 10000 hours or longer, according to a creep test on a notched test piece, under the same conditions as described above. The low-pressure part of the high pressure and low pressure integrated type turbine rotor has an excellent toughness of 0.2% yield strength of 686 MPa or more, and Charpy impact absorbed energy of 98 J or more. The high pressure and low pressure integrated type turbine rotor of the present invention has excellent creep properties at the high-pressure part and excellent toughness at the low-pressure part simultaneously.
The process for producing a high pressure and low pressure integrated type turbine rotor of the present invention is a method in which a rotor member made of an alloy steel having a specific composition is subjected to different heat treatments at its high pressure and low pressure parts, respectively. More particularly, the high pressure and low pressure integrated type turbine rotor of the present invention can be obtained by providing a rotor member made of an alloy steel having a specific composition, quenching the part corresponding to the high-pressure part of the rotor member after heating at a temperature of 980xc2x0 C. or more and 1100xc2x0 C. or less, cooling it at a higher cooling rate not lower than the air impact cooling rate while heating the part corresponding to the low-pressure part of the rotor member at a temperature of 850xc2x0 C. or more and less than 980xc2x0 C., and cooling it at a lower cooling rate not lower than the oil cooling rate. Thus, the part corresponding to the high-pressure part of the rotor member is quenched after heating to a high temperature and tempering it at a high temperature, while the part corresponding to the low-pressure part of the rotor member is quenched after heating to a relatively low temperature and tempering it at a relatively low temperature. Use of different heat treatments between the high-pressure and low-pressure parts can make the high-pressure part have excellent high temperature properties of a creep rupture time of 10000 hours or longer, according to a creep test on a notched test piece, under specific conditions of a temperature of 600xc2x0 C. and a stress of 147 MPa, and the low-pressure part have excellent toughness of Charpy impact absorbed energy of 98 J or more.
The specific alloy steel composition which can exhibit such excellent properties as above will be described in detail hereinbelow, but briefly it is characterized by allowances of contents of impurity elements such as phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony, which could affect adversely the embrittlement resistance at high temperatures of CrMoV based heat resistant steels and CrMoV based heat resistant steels containing tungsten, being limited to predetermined values or less.
First, of the high-temperature properties, the creep rupture strength of a notched test piece will be described. When a stress is applied to a steel product at a high temperature, even if the stress is comparatively small, the steel product plastically deforms very gradually to become elongated, and finally the elongation proceeds rapidly narrowing a part of the steel product, which results in rupture in the steel product. This phenomenon is called xe2x80x9ccreepxe2x80x9d or xe2x80x9ccreep rupture phenomenonxe2x80x9d. This phenomenon is believed to occur due to viscous flow at crystal grain boundaries and dislocation within crystals. In a high-temperature creep test, a constant static load is applied to a material for a long time at a high temperature, and the time elapsed before rupture is measured. As a test piece, a round bar having a constant cross section is used. The measuring method is defined by JIS Z-2272. The measuring methods defined by the JIS standards are for creep tests on unnotched test pieces, and test pieces which are finished by smoothly shaving between gauge marks in the portion to be measured are used in these methods.
In contrast, in a creep test on a notched test piece, a test piece having a notch between gauge marks is used. The cross section of the portion to be stretched and subject to measurement is set to be the same as the cross section of the part subject to the measurement in a creep test on an unnotched test piece, and the stress is determined. The diameter of the parallel part of the test piece (corresponding to the portion between gauge marks) is set to 1.2 times the diameter of the bottom of the notch, and the notch is formed so that it has an opening angle of 60xc2x0 and a radius of curvature of 0.13 mm at the bottom of notch, and is cut perpendicularly to the direction of drawing. In a creep test on an unnotched test piece, a tensile stress which is applied gradually elongates the distance between gauge marks, and narrows the portion between the gauge marks, which finally will rupture. In contrast, if a notch is formed in a test piece, a stress which counteracts deformation of the notched portion is produced such that the stress surrounds the notched portion (this stress is a so-called xe2x80x9cmultiaxial stressxe2x80x9d), and the test piece finally ruptures without being uniformly elongated. In general, with a highly ductile material, the lapse of time before rupture tends to be longer than that of the creep test on the unnotched test piece because deformation is restricted by the notch. However, depending on the type of steel, embrittlement of some materials gradually advances during the creep rupture tests, and a creep rupture may occur due to the occurrence of voids or the formation of cracks from connected voids. In this case, a notched test piece ruptures in a shorter time than an unnotched test piece due to the concentrated stress. Such a phenomenon is called xe2x80x9cnotch softeningxe2x80x9d, which can be used as an index for expressing creep embrittlement. That is to say, by conducting creep rupture tests on an unnotched test piece and a notched test piece under the same conditions such as stress and temperature, and comparing the times elapsed before creep rupture, the level of creep embrittlement can be clearly demonstrated.
Since a turbine rotor is subjected to high temperatures for a long period of time under stress during its operation, deterioration in the strength of the material with age is of concern. The quality of turbine rotor members has been hitherto evaluated only by high-temperature creep tests on unnotched test pieces, as defined by the Japanese Industrial Standards or the like. However, the present inventors have found a method of evaluating high-temperature strength properties of the material, particularly the creep embrittlement resistance, in a high-temperature creep test on a notched test piece. In addition, the present inventors have found that trace impurity elements which are harmful and greatly affect creep embrittlement. As a result, the present inventors succeeded in developing a material which can be quenched after heating to a high temperature of approximately 1000xc2x0 C. or more, which is inhibited from producing precipitation of a pro-eutectoid ferrite phase, and which is not subject to creep embrittlement, by minimizing the amount of trace impurity elements which are harmful, such as phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony.
Since the rotor is made of a CrMoV based heat resistant steel containing minimized amounts of harmful trace impurity elements and CrMoV based heat resistant steels containing tungsten, when the part corresponding to its high-pressure part is quenched after heating at a higher temperature of 980xc2x0 C. or more and 1100xc2x0 C. or less and tempered at a cooling rate not lower than the air impact cooling rate, excellent creep embrittlement resistance can be obtained. On the other hand, when the part corresponding to its low-pressure part is quenched after heating at a lower temperature of 850xc2x0 C. or more and less than 980xc2x0 C., and cooling it at a lower cooling rate not lower than the oil cooling rate, excellent toughness can be obtained.
That is to say, an alloy according to the first aspect of the present invention is a low-alloy heat-resistant steel comprising:
carbon in an amount of 0.20 to 0.35% by weight,
silicon in an amount of 0.15% by weight or less,
manganese in an amount of 0.05 to 1.0% by weight,
nickel in an amount of 0.3 to 1.5% by weight,
chromium in an amount of 1.0 to 3.0% by weight,
molybdenum in an amount of 0.5 to 1.5% by weight,
vanadium in an amount of 0.1 to 0.3% by weight,
phosphorus in an amount not larger than 0.012% by weight or substantially no phosphorus,
sulfur in an amount not larger than 0.005% by weight or substantially no sulfur,
copper in an amount not larger than 0.15% by weight or substantially no copper,
aluminum in an amount not larger than 0.01% by weight or substantially no aluminum,
arsenic in an amount not larger than 0.01% by weight or substantially no arsenic,
tin in an amount not larger than 0.01% by weight or substantially no tin, and
antimony in an amount not larger than 0.003% by weight or substantially no antimony,
the balance being iron and unavoidable impurities.
By limiting the permissible amounts of phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony impurities, which are harmful in causing creep embrittlement in conventional CrMoV steels, to low levels, the creep embrittlement resistance is particularly improved.
An alloy according to the second aspect of the present invention is a low-alloy heat-resistant steel comprising:
carbon in an amount of 0.20 to 0.35% by weight,
silicon in an amount of 0.15% by weight or less,
manganese in an amount of 0.05 to 1.0% by weight,
nickel in an amount of 0.3 to 2.5% by weight,
chromium in an amount of 1.0 to 3.0% by weight,
molybdenum in an amount of 0.5 to 1.5% by weight,
tungsten in an amount of 0.1 to 3.0% by weight,
vanadium in an amount of 0.1 to 0.3% by weight,
phosphorus in an amount not larger than 0.012% by weight or substantially no phosphorus,
sulfur in an amount not larger than 0.005% by weight or substantially no sulfur,
copper in an amount not larger than 0.10% by weight or substantially no copper,
aluminum in an amount not larger than 0.01% by weight or substantially no aluminum,
arsenic in an amount not larger than 0.01% by weight or substantially no arsenic,
tin in an amount not larger than 0.01% by weight or substantially no tin, and
antimony in an amount not larger than 0.003% by weight or substantially no antimony,
the balance being iron and unavoidable impurities.
Tungsten is added to the alloy according to the first aspect with the intention of improving particularly the creep rupture strength at the high-pressure part. Furthermore, as in the alloy according to the first aspect, by limiting the permissible amounts of phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony impurities, which are harmful in causing creep embrittlement, to low levels, the creep embrittlement resistance is particularly improved. Here, when importance is laid on the improvement in the creep rupture strength at the high-pressure part, the content of tungsten may be made larger to some extent while importance is laid on the improvement in toughness at the low-pressure part, the content of tungsten may be made smaller to some extent.
An alloy according to the third aspect of the present invention is a low-alloy heat-resistant steel comprising:
carbon in an amount of 0.20 to 0.35% by weight,
silicon in an amount of 0.15% by weight or less,
manganese in an amount of 0.05 to 1.0% by weight,
nickel in an amount of 0.3 to 2.5% by weight,
chromium in an amount of 1.0 to 3.0% by weight,
molybdenum in an amount of 0.5 to 1.5% by weight,
vanadium in an amount of 0.1 to 0.3% by weight,
cobalt in an amount of 0.1 to 3.0% by weight,
phosphorus in an amount not larger than 0.012% by weight or substantially no phosphorus,
sulfur in an amount not larger than 0.005% by weight or substantially no sulfur,
copper in an amount not larger than 0.15% by weight or substantially no copper,
aluminum in an amount not larger than 0.01% by weight or substantially no aluminum,
arsenic in an amount not larger than 0.01% by weight or substantially no arsenic,
tin in an amount not larger than 0.01% by weight or substantially no tin, and
antimony in an amount not larger than 0.003% by weight or substantially no antimony,
the balance being iron and unavoidable impurities.
Cobalt is added to a conventional CrMoV steel with the intention of improving the creep rupture strength at the high-pressure part and the toughness at the low-pressure part. Furthermore, by limiting the permissible amounts of phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony impurities, which are harmful in causing creep embrittlement, to low levels, the creep embrittlement resistance is particularly improved.
An alloy according to the fourth aspect of the present invention is a low-alloy heat-resistant steel comprising:
carbon in an amount of 0.20 to 0.35% by weight,
silicon in an amount of 0.15% by weight or less,
manganese in an amount of 0.05 to 1.0% by weight,
nickel in an amount of 0.3 to 2.5% by weight,
chromium in an amount of 1.0 to 3.0% by weight,
molybdenum in an amount of 0.5 to 1.5% by weight,
tungsten in an amount of 0.1 to 3.0% by weight,
vanadium in an amount of 0.1 to 0.3% by weight,
cobalt in an amount of 0.1 to 3.0% by weight,
phosphorus in an amount not larger than 0.012% by weight or substantially no phosphorus,
sulfur in an amount not larger than 0.005% by weight or substantially no sulfur,
copper in an amount not larger than 0.15% by weight or substantially no copper,
aluminum in an amount not larger than 0.01% by weight or substantially no aluminum,
arsenic in an amount not larger than 0.01% by weight or substantially no arsenic,
tin in an amount not larger than 0.01% by weight or substantially no tin, and
antimony in an amount not larger than 0.003% by weight or substantially no antimony,
the balance being iron and unavoidable impurities.
Tungsten and cobalt are added to a conventional CrMoV steel with the intention of improving the creep rupture strength at the high-pressure part and the toughness at the low-pressure part. Furthermore, by limiting the permissible amounts of phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony impurities, which are harmful in causing creep embrittlement, to low levels, the creep embrittlement resistance is particularly improved.
An alloy according to the fifth aspect of the present invention is a low-alloy heat-resistant steel comprising:
carbon in an amount of 0.20 to 0.35% by weight,
silicon in an amount of 0.15% by weight or less,
manganese in an amount of 0.05 to 1.0% by weight,
nickel in an amount of 0.3 to 1.5% by weight,
chromium in an amount of 1.0 to 3.0% by weight,
molybdenum in an amount of 0.5 to 1.5% by weight,
vanadium in an amount of 0.1 to 0.3% by weight,
at least one selected from the group consisting of niobium in an amount of 0.01 to 0.15% by weight, tantalum in an amount of 0.01 to 0.15% by weight, nitrogen in an amount of 0.001 to 0.05% by weight, and boron in an amount of 0.001 to 0.015% by weight,
phosphorus in an amount not larger than 0.012% by weight or substantially no phosphorus,
sulfur in an amount not larger than 0.005% by weight or substantially no sulfur,
copper in an amount not larger than 0.15% by weight or substantially no copper,
aluminum in an amount not larger than 0.01% by weight or substantially no aluminum,
arsenic in an amount not larger than 0.01% by weight or substantially no arsenic,
tin in an amount not larger than 0.01% by weight or substantially no tin, and
antimony in an amount not larger than 0.003% by weight or substantially no antimony,
the balance being iron and unavoidable impurities.
This alloy is intended to further improve the creep properties on an unnotched test piece with a view to increasing particularly the creep rupture strength at the high-pressure part by addition of at least one of trace elements selected from niobium, tantalum, nitrogen, and boron to the alloy according to the first aspect. Furthermore, as in the alloy according to the first aspect, by limiting the permissible amounts of phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony impurities, which are harmful in causing creep embrittlement, to low levels, the creep embrittlement resistance is particularly improved.
An alloy according to the sixth aspect of the present invention is a low-alloy heat-resistant steel comprising:
carbon in an amount of 0.20 to 0.35% by weight,
silicon in an amount of 0.15% by weight or less,
manganese in an amount of 0.05 to 1.0% by weight,
nickel in an amount of 0.3 to 2.5% by weight,
chromium in an amount of 1.0 to 3.0% by weight,
molybdenum in an amount of 0.5 to 1.5% by weight,
tungsten in an amount of 0.1 to 3.0% by weight,
vanadium in an amount of 0.1 to 0.3% by weight,
at least one selected from the group consisting of niobium in an amount of 0.01 to 0.15% by weight, tantalum in an amount of 0.01 to 0.15% by weight, nitrogen in an amount of 0.001 to 0.05% by weight, and boron in an amount of 0.001 to 0.015% by weight,
phosphorus in an amount not larger than 0.012% by weight or substantially no phosphorus,
sulfur in an amount not larger than 0.005% by weight or substantially no sulfur,
copper in an amount not larger than 0.15% by weight or substantially no copper,
aluminum in an amount not larger than 0.01% by weight or substantially no aluminum,
arsenic in an amount not larger than 0.01% by weight or substantially no arsenic,
tin in an amount not larger than 0.01% by weight or substantially no tin, and
antimony in an amount not larger than 0.003% by weight or substantially no antimony,
the balance being iron and unavoidable impurities.
This alloy is intended to further improve the creep properties on an unnotched test piece with a view to increasing particularly the creep rupture strength at the high-pressure part by the addition of at least one of trace elements selected from niobium, tantalum, nitrogen, and boron to the alloy according to the second aspect.
An alloy according to the seventh aspect of the present invention is a low-alloy heat-resistant steel comprising:
carbon in an amount of 0.20 to 0.35% by weight,
silicon in an amount of 0.15% by weight or less,
manganese in an amount of 0.05 to 1.0% by weight,
nickel in an amount of 0.3 to 2.5% by weight,
chromium in an amount of 1.0 to 3.0% by weight,
molybdenum in an amount of 0.5 to 1.5% by weight,
tungsten in an amount of 0.1 to 3.0% by weight,
vanadium in an amount of 0.1 to 0.3% by weight,
cobalt in an amount of 0.1 to 3.0% by weight,
at least one selected from the group consisting of niobium in an amount of 0.01 to 0.15% by weight, tantalum in an amount of 0.01 to 0.15% by weight, nitrogen in an amount of 0.001 to 0.05% by weight, and boron in an amount of 0.001 to 0.015% by weight,
phosphorus in an amount not larger than 0.012% by weight or substantially no phosphorus,
sulfur in an amount not larger than 0.005% by weight or substantially no sulfur,
copper in an amount not larger than 0.15% by weight or substantially no copper,
aluminum in an amount not larger than 0.01% by weight or substantially no aluminum,
arsenic in an amount not larger than 0.01% by weight or substantially no arsenic,
tin in an amount not larger than 0.01% by weight or substantially no tin, and
antimony in an amount not larger than 0.003% by weight or substantially no antimony,
the balance being iron and unavoidable impurities.
This alloy is intended to further improve the creep properties on an unnotched test piece with a view to increasing particularly the creep rupture strength at the high-pressure part by the addition of at least one of trace elements selected from niobium, tantalum, nitrogen, and boron to the alloy according to the fourth aspect.
The high pressure and low pressure integrated type turbine rotor of the present invention has high temperature creep properties, and particularly exhibits excellent creep properties on a notched test piece and excellent toughness simultaneously. The high-pressure part of the high pressure and low pressure integrated type turbine rotor has excellent high temperature properties with a creep rupture time of 3000 hours or longer, according to a creep test on an unnotched test piece, under specific conditions of a temperature of 600xc2x0 C. and a stress of 147 MPa, and a creep rupture time of 10000 hours or longer, according to a creep test on a notched test piece, under the same conditions as described above. The low-pressure part of the high pressure and low pressure integrated type turbine rotor has an excellent toughness of 0.2% yield strength of 686 MPa or more, and Charpy impact absorbed energy of 98 J or more. The high pressure and low pressure integrated type turbine rotor of the present invention has a creep embrittlement index of 1.6 or more, preferably 2.0 or more, and more preferably 3.0 or more, wherein the index is defined by a ratio of a creep rupture time in a creep rupture test on a notched test piece to a creep rupture time in a creep rupture test on an unnotched test piece.
The high temperature creep property is judged by the length of creep time on an unnotched test piece and in addition by the creep embrittlement index in order not to cause creep embrittlement. To cause no creep embrittlement, a creep embrittlement index of 1.5 is unsatisfactory and at least 1.6 is necessary. The turbine rotor having a creep rupture time exceeding 10000 hours has a creep embrittlement index exceeding 1.6 and even a turbine rotor having a creep embrittlement index exceeding 3.0 can also be realized.
As explained above, a high pressure and low pressure integrated type turbine rotor having an excellent creep rupture strength and an excellent toughness has been provided by the present invention for the first time.
Further, the process for producing a high pressure and low pressure integrated type turbine rotor according to the present invention is to heat a turbine rotor member made of each alloy steel containing the above specific components at a temperature of 980xc2x0 C. or more and 1100 xc2x0 C. or less at a part corresponding to the high-pressure part of the turbine rotor member, cooling it at a cooling rate higher than the air impact rate while heating the part corresponding to the low-pressure part of the turbine rotor member at 850xc2x0 C. or more and less than 980xc2x0 C., and cooling it at a cooling rate higher than oil quenching rate.
The heating of the part corresponding to the high-pressure part of a turbine rotor at high temperatures is intended to have the alloy elements dissolved in the alloy matrix sufficiently and make crystal grains relatively coarse to impart high temperature strength thereto. On the other hand, the heating of the part corresponding to the low-pressure part of a turbine rotor at temperatures lower than the temperature of the high-pressure part is intended to make the crystal grains finer in order to increase toughness.
The high pressure and low pressure integrated type turbine rotor of the present invention has excellent high temperature strength and excellent creep rupture strength at its high-pressure part and excellent mechanical strength and toughness at its low-pressure part simultaneously so that it can be used at higher temperatures in a large volume steam turbine, thus enabling realization of an electric power plant having a high energy efficiency and being extremely useful.
According to the process for producing a high pressure and low pressure integrated type turbine rotor of the present invention, a turbine rotor that is free of creep embrittlement even when it is quenched after being heated at a high temperature in the range of 980xc2x0 C. or more and 1,100xc2x0 C. or less at its high-pressure part can be obtained easily by minimizing the contents of harmful impurity elements.
Also, a turbine rotor can be obtained easily which is excellent in 0.2% yield strength and has a high Charpy impact value and excellent toughness at its low-pressure part.